Improvement in the catalyst of olefin polymerization has enhanced a drastic increase in the production capability of olefin polymers in recent years, and the operation to remove the catalyst after polymerization can be eliminated. When applying that type of high activity catalyst, gas-phase olefin polymerization is commonly applied from the standpoint of simple operation after the polymerization.
That type of gas-phase polymerization widely adopts a gas-phase fluidized-bed reactor utilizing gas-solid fluidized bed. The gas-phase fluidized-bed reactor usually has a plate having a large number of holes, (hereinafter referred to as the “gas-distribution plate”), at the lower part of the reaction vessel. The raw material gas enters the reaction vessel through the gas-distribution plate to conduct the polymerization reaction in the fluidized bed to which the polymerization catalyst is introduced. The olefin is polymerized in the presence of the polymerization catalyst to generate the olefin polymer peripheral to the polymerization catalyst, and the olefin polymer particles are formed. Thus formed olefin polymer particles are then withdrawn from the withdrawal port located at the lower part of the reaction vessel.
A common reaction vessel for the above gas-phase fluidized-bed reactor is a vertical reaction vessel having a right circular cylindrical shape. Alternatively, there is a known reaction vessel, having a structure of gradually increasing inner diameter thereof in an ascending direction, for the purpose of preventing scattering of the olefin polymer particles from the fluidized bed against the inner walls at the upper part of the reaction vessel (for example, refer to Patent Document 1).    [Patent Document 1] Japanese Patent Application Laid-open No. 10-279612